More information about the eye

The External Eye

The front of the eye, except the cornea, is covered by a lining called the conjunctiva, which passes from the eyeball onto the back of the eyelids, and becomes continuous with the skin of the face. At this point emerge eyelashes, which protect the eye from dust, each with a tiny sebaceous gland.

Tears consist of watery fluid which lubricates the front of the eye and helps to prevent infection. They are produced continuously by glands in the upper outer part of the eyelid - the lacrimal glands - and they drain through two tiny holes, the puncta, at the inner end of the eyelids into the nose, which is why our nose runs when we cry. Blinking helps the fluid to drain.

The Front Internal Eye

At the front of the eye the white sclera is modified to be transparent, the cornea. Behind this there is a circular gap in the choroid: this is the pupil, which can be made smaller or larger by the action of two muscles depending on the amount of light, or whether we're looking at something close up, or emotion like fear.

The eyeball is divided into two parts by the lens. The larger part behind the lens is filled with a clear, jelly - like substance, the vitreous humour, while the smaller part in front of the lens is filled by a watery fluid, the aqueous humour.

Light passing through the cornea and the pupil enters the lens, which is made up of clear protein contained in a capsule. The shape of the lens can be altered by a circular muscle in order to focus the light on the retina - a process called accommodation. This enables us to see things clearly both close up and at a distance.

The Rear Internal Eye

The eyeball has three layers: an outer, fibrous layer called the sclera; a coloured middle layer, the choroid, (which at the front becomes the iris, that gives the colour to our eyes), and an inner layer, the retina, which is made up of nerve cells which respond to light. There are two types of cells - rods (about 125 million) which respond to dim light, and cones (about 6 million) which respond to colour, either red, blue or green.

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